1. Field of the Invention
The invention is in the field of wavelength measurements utilizing fringe counting techniques in a Michelson-type interferometer. The invention is particularly directed toward rapid and convenient measurement of cw laser radiation.
2. Description of the Prior Art
Fringe counting techniques in interferometers have long been utilized to provide an accurate determination of distances by comparing the unknown distances with the wavelength of a known source. Various optical arrangements for a Michelson interferometer including those utilizing corner-cube reflectors are disclosed in the article by W. R. C. Rowley, "Some Aspects of Fringe Counting in Laser Interferometers", IEEE Transactions on Instruments and Measurements, Vol. IM-15, pages 146-149, 1966. A traveling Michelson interferometer for laser wavelength determination was described by B. L. Bukovskii et al in Pribory i Teknika Eksperimenta, No. 3, pp. 175-177, May-June 1974 (translated by Consultants Bureau, Plenum Press, NY as UDC 621,317,365). Measurement techniques are also known utilizing phase-lock frequency multiplication to enhance the Doppler shift in corner-cube interferometers as discussed in the article entitled "Angstrom Measurements With Velocity-of-Light Compensation, The Remote Interferometer, and a Simple Electronic Resolution Extender", Hewlett-Packard Journal, December 1971, page 18. In this system, the basic frequency is defined by an intermode beat frequency of the reference laser and contains the influence of the interferometer only as a perturbation. Additionally, since this system is designed to measure displacement, only one laser wavelength is employed in its interferometer.
With the advent of the cw dye laser which may be continuously tuned over several Doppler absorption line widths, and with its application to sub-Doppler spectroscopy, it is desirable to rapidly determine the wavelength of the laser. On-line wavelength determinations with rapid updating is particularly desirable in two-photon Doppler-free spectroscopy inasumch as one is looking for weak narrow lines with no linear fluorescence.